High frequency amplifier



Jan. 22, 1935. H. sTOET ET AL HIGH FREQUENCY AMPLIFIER Filed June 2, 1953 INVENTORS Hi'ndril SZoeZ' and BY LouisJfiWanHarreveZi' ATTORNEYS Patented Jan. 22, 1935 ATENT OFFICE HIGH FREQUENCY AMPLIFIER Hindrik Stoet, Voorburg, near The Haguaand Louis Henri van Harrevelt; The Hague, Nethe erlands, assignors to Technisch Handelsbureau Rune, The Hague, Netherlands, a privat Dutch company of the Netherlands;

,,Application June 2, 1933, Serial No. 674,118 V In the Netherlands June 6, 1932 1 Claim. (Cl. 179-171) The invention relates to a high frequency amplifier. for equallyamplifying an extended range of high frequencies.

Heretofore ithas not been possible to realize bymeans of the known high frequency amplifiers a constant amplification over an extended frequency range. The parasitic capacities of the circuit produce a decrease of amplification for 1 thechigher-frequencies.

drawback is removed by adopting the circuit according to the invention. It is an object of the invention to provide a high frequency choke coil amplifier comprising at least two vacuum tubes and producing an even amplification over a great frequency range.

According to the invention the choke coil for coupling the tubes is devised in such a way that the inductive impedance of this coil for the lowest frequency to be amplified is practically the same as the capacity impedance of the total active parasitic capacity of the circuit for the highest frequency to be transmitted, whereas the resistance provided for applying a biassing voltage to the control electrode of the second tube is of such value that in the middle of the frequency range transmitted practically the same amplification occurs as at the limits of this range.

The parasitic capacity of the coupling circuit is composed of the anode capacity of the first tube, the distributed capacity of the coupling coil, the grid resistance of the next tube and the conductors connected thereto, and further of the grid-cathode capacity and the grid anode capacity of the second tube. The latter capacity will be subject to an apparent increase under working conditions of the amplifier in a manner known per se due to the alternating voltage on the anode of the second tube. The value of this increase depends upon the characteristics of the tube employed and upon the arrangement of the anode circuit thereof.

When the coupling coil and the grid resistance are selected in the manner indicated above, an amplifier will be obtained having a substantial constant amplification over an extended frequency range, whereas at the limits of this frequency range a favorable decrease of amplification will be produced. The frequency range transmitted may for instance comprise the long as well as the short broadcast wave band. Due to the decrease of amplification beyond the limits of this frequency range all frequencies interfering with broadcast reception will be substantially suppressed.

The amplifier according to the invention may be used for any purpose and under all conditions where it is-necessa-ry to produce an even ampli-. fication of an extended rangeof. high frequencies. It is of particular importance in connection with a central antenna system serving for supplyinga great number of wireless receivers withhighi frequency energy. In such a system the oscilla tions of all frequencies received by the antenna will first be aperiodically amplified and, after that, will be transmitted over conductors to the individual receivers. The advantages of such a distributing system are known. In a system of this type it is of importance that all broadcast stations transmitting for instance in a frequency range of 800 to 900 kilocycles (short wave band and long ,wave band) are equally amplified whereas all interfering stations operating outside this range are suppressed as much as possible. For this purpose the amplifier according to the invention may be employed.

The invention will be explained in connection with a modification shown by way of example in the drawing.

In this drawing, 1 is the first tube, 2 the second tube, 3 the coupling coil, 4 a resistance by means of which the grid of the second tube receives a biassing voltage, and 5 a condenser preventing the anode voltage of the first tube from being applied to the grid of the second tube. The circuitarrangement, shown is that of an ordinary high frequency choke coil amplifier.

shunted across the coupling coil 3 is a parasitic capacity 6 and shunted across the input of the tube 2 a parasitic capacity 7. The parasitic capacity 6 comprises the anode capacity of the tube 1 and the distributed capacity of the coupling coil 3 and the conductors connected thereto.

The parasitic capacity 7 is composed of the distributed capacity of the resistance 4 and the conductors connected thereto, and further of the grid-cathode capacity and the grid-anode capacity of the tube 2. On operation of the amplifier the latter capacity will undergo an increase due to the fact that alternating voltages occur on the anode of the tube 2 and produce a certain amount of reaction on the grid of this tube. The capacity of the condenser 5 may be left out of consideration as it may be given such a value that compared with the other capacities it constitutes a short circuit for all frequencies. Under this condition all parasitic capacities may be considered to be connected in parallel.

According to the invention the coupling coil 3 is selected in such a way that the inductive impedance of this coil for the lowest frequency to be transmitted is equal or substantially equal to the capacitive impedance of the combination of all parasitic capacities for the highest frequency to be transmitted. Then the same amplification will be produced at both limits of the total frequency range. This amplification may also be obtained in the middle of the frequency range when the value of the resistance 4 is suitably ad- J'usted. Below the lowest frequency and above the highest frequency a decrease of amplification will be produced, the effect of which is that frequencies situated outside the frequency range to be amplified are substantially suppressed.

The advantages of the constant amplification of the desired frequency range and the suppression of all frequencies outside this range are of considerable importance when the amplifier comprises more than two tubes. For in this case a multiplication of the amplification values of the individual stages is obtained. When more than two tubes are used any pair of succeeding tubes may be arranged as shown in the drawing.

The total parasitic capacity which, as has been shown above, limits the amplification of the highest frequency may be reduced by keeping low the distributed capacities of all parts of the circuit and by suitable selection and construction of the tube following the coupling coil,

The resistances 8 and 9 shown in the drawing and the condensers arranged in parallel therewith are providedto produce a suitable grid bias for the tubes 1 and 2.

The cathodes of the tubes are of the indirectly heated, type. It will be clear, however, that this feature as well as the further construction of the tubes is not essential for the invention. In

general any suitable kind of tube may be employed.

' What we claim is:-

An electric amplifier comprising at least two tubes coupled by means of a choke coil arranged for equally amplifying an extended range of high frequencies, characterized in that the coupling coil is of such dimensions that the inductive impedance of said coil for the lowest frequency is substantially equal to the capacitive impedance of the'active parasitic capacity of the circuit for the highest frequency of the frequency range to be amplified, the resistance supplying a biasing voltage tothe control electrode of the second tube being of such a value that its impedance for frequencies in the middle of said frequency range is substantialy equal to the inductive impedance of the coil for the lowest and the capacitive impedance of the capacity for the highest frequency of the range.

' HINDRIK STOET.

LOUIS HENRI van HARREVELT. 

